A blasting method of this type has been disclosed in U.S. Pat. No. 5,616,067 A. The CO2 is introduced in liquid form into an annular chamber which surrounds the blasting line through which compressed air is passed, and from there the CO2 is fed into the blasting line through a circular array of converging capillaries, so that the expansion occurs only upon entry into the blasting line. The dry snow thus created is entrained and accelerated by the compressed air and is jetted onto the workpiece to be cleaned via the blasting nozzle. This method is particularly intended for gently cleaning pressure-sensitive surfaces in such as electronic circuit boards.
U.S. Pat. No. 5,679,062 describes a blasting method in which gaseous or liquid CO2 or a mixture of gas and liquid is expanded at the outlet of a nozzle and is introduced into an enlarged vortex chamber in which a part of the gaseous and/or liquid CO2 is transformed into dry snow. The outlet of the vortex chamber is directly coupled to the blasting nozzle. Here, the carrier gas is formed by the gaseous CO2 that has been supplied or is produced through evaporation.
U.S. Pat. No. 5,725,154 A describes a blasting method in which dry snow is produced by expanding liquid CO2 by means of an expansion valve. Through a thin tube which is coaxially surrounded by a tube for supplying the carrier gas, the dry snow is supplied to a blasting pistol which then jets out in a mixture of carrier gas and dry snow.
WO 00/74 897 A1 discloses a blasting apparatus in which liquid CO2 is supplied via a capillary which opens into a conically divergent nozzle the diameter of which increases towards the outlet to approximately three times the diameter of the capillary. This nozzle is surrounded by an annular Laval nozzle in which the carrier gas that has been supplied under pressure is accelerated to supersonic speed. The mouths of the CO2 nozzle and the Laval nozzle are level with one another, so that two concentric jets are produced, i.e. an inner jet consisting mainly of dry ice and a jacket jet which is to accelerate the dry ice outside of the nozzle.
Also in applications in which larger surfaces such as the internal surfaces of pipes or boilers in industrial equipment shall be freed of firmly adhering incrustations, the use of dry ice or dry snow as blasting material, depending on the type of incrustations, is frequently desirable, because the low temperature of the dry ice or dry snow makes the material to be removed more brittle. When particles of dry snow penetrate into the layer to be removed with sufficient kinetic energy, a cleaning effect is achieved by the fact that the particles of dry snow, when penetrating into the layer to be removed, are evaporated abruptly and thus blow off parts of the layer to be removed. Another advantage is that no additional means are necessary for discharging the used blasting material, because the dry snow evaporates to gaseous CO2.
However, the blasting methods described above are not suitable for these kinds of application, because the achievable volume flow rates and jet speeds are not sufficient and/or because dry snow is not produced in a sufficient amount or does not have the correct composition, so that the kinetic energy of the particles of dry snow is to small.
For this reason, for cleaning larger, heavily contaminated surfaces, blasting equipments have heretofore been used in which dry ice or dry snow is stored in solid form in suitable cooling tanks and is metered into the flow of compressed air. The compressed air and the dry snow serving as blasting material are then delivered together through a pressure hose which connects the blasting equipment to the blasting nozzle. However blasting methods and apparatus of this type require cumbersome installations and correspondingly high equipment costs as well as high expenses for storing the dry snow.